The present invention relates to a method for uplink data transmission in a Long Term Evolution (LTE) compliant communication system which comprises a base station and at least one wireless communication terminal. The invention further relates to a wireless communication terminal and to a base station in a Long Term Evolution (LTE) compliant communication system, in particular employing a frequency division duplex (FDD) transmission scheme.
Cellular systems are used to offer wireless telephony and data services to their users. The new cellular standard developed by the 3rd generation partnership program (3GPP) called Long Term Evolution (LTE) offers unprecedented data rates and unprecedented shortest latency to the end customer while promising a high spectral capacity for the network operator. This allows a network operator to make best use of the available spectrum while at the same time providing internet experience similar to wired internet to the end-customer.
An important parameter for any wireless system is the received energy per bit and the corresponding signal-to-noise ratio (SNR). For a fixed transmit power, obviously, as the bit rate increases, the energy per bit decreases. In cellular communications systems, uplink (from a terminal to a base station) and downlink (from a base station to a terminal) transmit power is typically asymmetric. Base stations can transmit with virtually any output power that the system needs for providing good coverage. For mobile terminals, residential gateways and the like, on the other hand, transmit power is limited due to implementation cost, power consumption and regulatory reasons. Therefore, cellular communications tend to be uplink limited, i.e. the coverage area is determined by the uplink rather than the downlink. This is true for any cellular system be it for mobile or fixed wireless access.
Currently, network operators plan to use LTE technology to provide wireless broadband access to houses, in particular in rural areas. For rural areas, however, as the population density is relatively low, providing a good coverage is crucial for lowering network deployment cost.
The LTE system, as already its predecessor UMTS, can adapt the modulation and bit rate to the channel conditions. A user that is far from a base station needs to transmit with a significantly higher energy per bit than a user that is nearby. As the maximum transmit power is limited, increasing the transmitted bit energy can be achieved by either lowering the data rate or by using a more energy efficient modulation scheme, e.g. QPSK instead of 16-QAM or a combination of both methods. The drawbacks of this are that an energy efficient modulation is not bandwidth efficient, i.e. a user that is located at the cell edge requiring an energy efficient modulation scheme consumes significantly more bandwidth than a less energy efficient albeit more bandwidth efficient technique would take. The bit rate can of course be lowered but only to the extent that the bit rate rests above a certain minimum that is acceptable by the user, and that the network has set itself as a minimum quality of service (QoS) limit. For symmetric traffic like voice over IP (VoIP) sacrificing uplink capacity can be more difficult. Therefore, a good uplink performance is crucial for cell coverage and likewise for uplink capacity. Larger cell coverage on the other hand can substantially reduce network deployment cost as less base stations need to be deployed.
As discussed above, the maximum transmitter output power is limited for mobile terminals and residential gateways and is substantially less than the limits for base stations defined by the regulator.
For the purpose of the invention the terms “terminal” and “user equipment” (UE) are intended to refer to any device used directly by an end-customer for wireless communication, such as a hand-held telephone, a communication device in a vehicle or in a laptop computer, a location fixed residential gateway or the like. The UE connects to the base station to provide wireless communication for the user.
Beam-forming techniques are known to enhance downlink performance of cellular networks. Beam-forming is a technique in which multiple antennas transmit the same signal with a phase offset. By doing so, the signals radiated from the different antennas add constructively or destructively depending on the phase of the signal with respect to the transmission angle. By carefully controlling the phase of the antennas, a beam can be formed, where the term “beam” typically refers to the direction into which signals add constructively. In the direction of the beam, the gain can be as high as N, where N is the number of antenna elements.
As an example, consider the use of four equal omni-directional antennas with equal output power. The total emitted output power will be four times the output power of one antenna. If the phases are correctly adjusted such that all signals add constructively for one specific direction, a beam is formed. The output power in the direction of the beam will be 16 times higher compared to a single isotropic radiating antenna element.
Beam-forming techniques are known and have been deployed for years, e.g. for radar. These techniques are often based on antenna arrays, as shown in FIG. 1. An antenna array consists of two or more antennas (10-1, 10-2, . . . , 10-n) that are located on a geometrical grid. Beam-forming is achieved by sending or receiving the same signal with a different phase. For narrow band signals, a phase shifter (12-1, 12-2, . . . , 12-n) is used. A beam is formed in directions where the individual signals from each antenna add constructively. Since the behavior of the channel is similar for both directions, the same also holds true for the reception of an RF signal.
The technique was first used in military and radar communications where mechanical, i.e. rotating antennas were replaced with a phased antenna array to steer the beam electronically rather than mechanically. As an alternative to analog phase shifters, phase shifting can also be applied in the digital domain. In this case, separate RF signals with a phase shift already applied are generated and fed to each transmitting antenna, individually. Again, by varying the phase, different beam patterns can be generated.
FIG. 2 shows an antenna pattern for two dipole antennas with half lambda spacing and zero degree phase shift, where lambda is the wavelength. The two transmit antennas are located at the horizontal axis. It can be seen from FIG. 2 that the two transmit signals add constructively along the vertical axis. In fact, a 6 dB gain over a single dipole antenna can be observed. As the spacing between the two antennas is one-half the wavelength, the signals of the two antennas add destructively along the horizontal axis if no phase shift is applied.
By applying a relative phase change to one of the antennas with respect to the other, the pattern can be changed. FIG. 3 shows the antenna pattern for the same antenna configuration but with a phase shift of 180 degrees applied to one of the two antennas. Now, the signals add constructively along the horizontal axis and destructively along the vertical axis. Again, a maximum gain of 6 dB over a single dipole antenna can be observed.
Usually, the antennas in such arrays are spaced by lambda/2. A smaller spacing of the antennas reduces the gain from interference and hence the yield of the array. With a larger spacing than lambda/2 so called grating lobes occur, in other words, more than one direction is preferred, wherein such grating lobes either present a gain. Generally, antenna arrays with lambda/2 spacing are preferred in the art, but other configurations are feasible. Details on beam-forming with antenna arrays are widely found in literature.
Beam-forming techniques are now also used in cellular technology. Third generation cellular phone standards UMTS and HSDPA as well as the new standard LTE support beam-forming in the downlink, i.e., from the base station to the mobile. The mobile phone supports these techniques by providing feedback information to the base station which then can adjust the beam accordingly. Moreover, beam-forming is used to adapt antennas of base stations e.g. to geographical or traffic conditions or local requirements.
LTE is the latest cellular standard defined by the 3rd generation partnership program (3GPP). It uses orthogonal frequency division multiple access (OFDMA) in the downlink and single carrier frequency division multiple access (SC-FDMA) in the uplink. Information blocks are placed in transport blocks. When the base station has granted transmission for a specific subframe, the terminal sends exactly one subframe. The subframe with a duration of 1 ms consists of two slots of 0.5 ms each. For uplink transmissions, it is possible for the base station to assign two frequencies blocks to the terminal, one to be used during the first slot of a subframe and the other to be used for the second slot of the subframe.
In the current revision of the LTE standard, i.e. Release 8, beam-forming techniques are supported in the downlink. In order to reduce interference to other base stations LTE makes use of uplink power control, like already the UMTS technology. Power control information is sent within every subframe. The base station controls uplink transmit power of a terminal with the aim to receiving a respective user equipment signal with the bare minimum signal strength required for error-free detection of the signal. In case the signal strength is too low, LTE uses a hybrid automated repeat request (HARM) technique to combine the information of a previous transport block with a repeated version. Additionally, if conditions persist, the base station requests the user equipment to increase its power.
Furthermore the current revision 8 of LTE standard assumes that a terminal may have more than one transmit antenna but only one power amplifier. The base station can trigger the terminal to switch to another transmit antenna via downlink control channel signalization.
In order to optimize the utilization of cell capacity the base station needs to gather some information about the uplink channel quality from each terminal connected to that base station. The individual channel quality may vary in function of the frequency. In TDD systems, channel quality information can be derived by exploiting channel reciprocity provided that the same antennas are used for transmit and receive.
However, in FDD systems, quality information for data transmissions can only be derived for subbands allocated to a terminal. Hence, an additional mechanism has been introduced to LTE uplink. Terminals are configured to periodically send sounding reference signals (SRS). The base station performs measurements on SRS originating from different terminals, and schedules terminals such that service quality and cell utilization are optimized.
It is an object of the present invention to provide apparatus and methods for increasing the range of LTE user equipment.